Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to the field of polycrystalline diamond compact (PDC) insert drill bits used to drill wellbores through earth formations. More specifically, the invention relates to selected arrangements of PDC cutting elements on such drill bits to improve drilling performance.
2. Background Art
Polycrystalline diamond compact (PDC) insert drill bits are used to drill wellbores through earth formations. PDC bits generally include a bit body made from steel or matrix metal. The bit body has blades or similar structures in it to which are attached a plurality of PDC cutting elements in a selected arrangement. The way in which the blades are structured, and the way in which the PDC cutting elements are arranged on the blades depend on, among other factors, the type of earth formations to be drilled with the particular PDC bit and the structure of a drilling assembly (known as a bottom hole assemblyxe2x80x94xe2x80x9cBHAxe2x80x9d) to which the drill bit is attached.
One feature of the arrangement of the cutting elements is known as the xe2x80x9cbackrakexe2x80x9d angle. This is an angle subtended between the plane of the cutting face (diamond table) of the PDC cutting element and a line parallel to the longitudinal axis of the drill bit, or perpendicular to the profile of the bit. Typically, PDC drill bits are designed so that the cutting elements have a relatively low backrake angle. Low backrake angle provides the drill bit with relatively high performance, by reducing the weight on bit (WOB) required to fail a given earth formation, meaning that rates of penetration through earth formations are high. However, low backrake angle increases the risk that the cutting elements will be subjected to impact damage, which normally appears as chipping or fracturing of the diamond table on the cutting elements, having the cutting elements break off the bit body, or otherwise prematurely and catastrophically fail. Another feature of low backrake angle is that wear flats which ultimately form on the cutting elements have a very large areal extent across the cutting element.
Several types of PDC bits known in the art include different backrake angles on the same bit in attempts to reduce cutting element wear and damage, while maintaining the relatively good performance provided by low backrake angle. One type of PDC bit known in the art includes cutting elements having backrake angle that increases with respect to the lateral or radial position of each cutting element with respect to the longitudinal axis of the bit. Typically, such bits have the cutting elements segregated into at least two groups of cutting elements. The first such group is located laterally inward, approximately from the longitudinal (bit) axis to a first selected radial extent. Cutting elements in the first group typically have a relatively low backrake angle, because these cutting elements are closer to the axis of the bit and as a result have smaller moment arms and do not create high torque. A second group of cutting elements starts at the radial limit of the first group and extends to the gage radius of the bit. Cutting elements in the second group have a higher backrake angle than those in the first group, because their moment arms are bigger. At higher backrake angles, the elements in this second group will have lower resulting forces, which helps to reduce the torque they will have created due to their bigger moment arms. Still other bits having this general arrangement of PDC cutting elements include a third group of cutting elements having higher backrake angle than the second group. The third group of cutting elements starts at a radial limit of the second group and continues out to the gage radius of the bit. Generally speaking, this type of PDC bit has increasing cutting element backrake angle as the radial distance of the cutting element increases. Increased backrake angle is usable because they make the cutting elements comparatively more passive, and thus less susceptible to impact damage in events of vibration behavior.
Low backrake angles in general improve the penetration rates of PDC bits. However, low backrake angles also reduce the amount of useable diamond on a PDC cutting element, and thus the bit""s life or durability. High backrake angles reduce rates of penetration, but cutting elements in such configurations are less susceptible to impact damage and present more useable diamond and thus improve bit life.
Another type of PDC bit known in the art includes PDC cutting elements having a first backrake angle on selected blades, and PDC cutting elements having a second backrake angle on other selected blades. Typically the selected backrake angle will alternate between successive blades.
The backrake arrangements known in the art, however, have not proven to be very suitable for use with high speed drilling tools and/or assemblies. Such drilling tools or assemblies, as known in the art, include xe2x80x9cturbinesxe2x80x9d hydraulic motors, and sometimes high rotary speed assemblies. What is needed, therefore, is a drill bit having cutting elements arranged to improve performance when used with high speed tools or drilling assemblies, especially turbines What is also needed is a bit which combines the increased life characteristics of high backrake angle with the increased rates of penetration associated with low backrake angle.
One aspect of the invention is a drill bit which includes a bit body having a plurality of blades thereon. The blades have a plurality of cutting elements affixed to them at selected positions. The cutting elements are disposed into at least two groups. A first one of the groups has at least 60 percent of its cutting elements disposed at a first mean backrake angle. A second group has at least 60 percent of its cutting elements disposed at a second mean backrake angle. The second mean backrake angle is at least about fifteen degrees more than the first mean backrake angle. The bottom hole coverage of the cutting elements in the second group is at least about eighty percent.
In some embodiments, each cutting element on the bit has a unique radial position with respect to the bit geometric axis. In some embodiments, the cutting elements in the second group have a higher abrasion resistance than the cutting elements in the first group. In some embodiments, each of the cutting elements has a backrake angle which is related to the radial distance of the cutting element from the bit axis.
In some embodiments, at least one cutting element is disposed at substantially the same radial position as a corresponding cutting element in either the first group or the second group. In some embodiments, the at least one cutting element has the same backrake angle as the corresponding cutting element. In some embodiments, the at least one cutting element is a different diameter than the corresponding cutting element. In some embodiments, at least one of the blades on the bit body has at least one cutting element from the first group and from the second group, and has at least one alternation of backrake angle thereon.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.